This invention relates to packaging of electrical circuits.
In certain packaging applications, e.g., within a modular DC--DC converter, heat from power dissipating components must be efficiently conducted to a metal outer surface of an enclosure (e.g., a baseplate). The metal outer surface must also be electrically insulated from the power dissipating components.
One way of packaging a power device, described in U.S. patent application Ser. No. 07/914,347, filed Nov. 23, 1993, assigned to the same assignee as this application and incorporated by reference, is illustrated in FIG. 1a. In FIG. 1a, a semiconductor die 10 is connected, (e.g., with solder (not shown)), to a conductive copper lead 14. Copper lead 14 may serve as an electrical connection to the die and may also serve as a heat spreading layer that helps in removing heat from die 10. Copper lead 14 is directly bonded through a eutectic or oxide bond 16 to a ceramic layer 18. An adhesive 20 (e.g., epoxy) holds ceramic layer 18 to an aluminum baseplate 22. This arrangement aids in conducting heat H, from semiconductor die 10, to baseplate 22. Ceramic layer 18 electrically insulates conductive copper lead 14 from conductive baseplate 22.
Another way of achieving both electrical insulation and thermal coupling between circuit components and a metal surface is by means of a Denka plate. A Denka plate is a composite laminate comprising alternating layers of thin metallic conductors (e.g., copper) and epoxy which is deposited on a relatively thicker metal plate. The epoxy layers provide insulation between the conductive layers and the metal plate.
It is well known to reduce the capacitance between a component and a metal baseplate by interposing a properly referenced electrostatic shield between the component and the baseplate. For example, FIG. 1b shows one way of providing a shield between a TO-220 power semiconductor package 200 and a metal baseplate 22. In FIG. 1b, the metal tab 250 of a TO-220 package is mounted to the top surface of a sandwich formed of two pieces of ceramic 218a, 218b and a metallic shield 230. The bottom surface of the sandwich is mounted (e.g., by epoxy layer 220) to a baseplate 22. Assume, for example, that a high frequency signal is present on the tab 250 and that the lead 240 and the baseplate 22 are at ground potential. If, as indicated in FIG. 1c, the lead 240 and the shield 230 are connected together (either directly, as shown, or via a capacitor, not shown), the current, In, which is capacitively coupled into the shield 230 via the tab-to-shield capacitance 260, Cps, will be returned to the grounded source lead 240 via the shield 230; little or no current will be coupled into the baseplate 22. On the other hand, if the shield is not present, or is not connected, current will be capacitively injected directly into the baseplate. In the latter case substantially greater EMI/RFI reduction measures might have to be taken to control conducted noise.